Paper can be manufactured from a variety of materials. Rags, for example, are used for the highest quality paper. Lower quality paper can be made from seed fibers, jute, flax, grass and other plants. The largest amount of paper today, however, is made from wood pulp.
Wood pulp manufacture typically begins with cutting trees, trimming the cut trees into logs and de-barking the logs to provide the raw wood material. The logs are sliced and the slices broken into chips. The chips are screened or torn to obtain a plurality of chips of a given size. This is often accomplished by delivering the wood chips to a refiner wherein the chips are torn to provide a raw fibrous pulp containing fibers of a desired length and size. The raw pulp is oftentimes washed and bleached to remove impurities. The purified, but yet raw, pulp is then delivered to a second refiner or a series of refiners that beat the pulp to a desired degree. One skilled in the art will appreciate that the refining process affects the length of the fibers, their plasticity and their capacity for bonding together in the papermaking machine. The quality of the finished product is determined more at the time of refining than at any other time in the paper production process. At the conclusion of the refining process, the pulp "stock" is suitable for introduction into a papermaking machine.
Different types of refiners are known. Some refiners are provided with cone-shaped beater rolls in a similarly shaped housing. Other refiners are provided with substantially round discs. Disc refiners typically include a mass chamber having an inlet for the incoming unrefined pulp material and an outlet for the refined pulp. Some disc refiners provide one rotating and one stationary disc, whereas others provide two rotating discs. The discs face one another and, through rotation of one or both discs, frictionally engage the stock to refine the pulp. U.S. Pat. No. 3,984,057, for example, describes a refiner with three coaxial discs; the outer discs are stationary and the intermediate disc is rotated by a power-driven shaft.
The pulp-engaging face of any refining disc is conventionally provided with a plurality of spaced blades, each of a predetermined thickness, height, and angular position. Dams are conventionally provided within the spaces so as to better process the stock. The arrangement of the blades and dams is, in part, dictated by the type of wood to be processed (i.e. hard, soft or otherwise) as well as by the desired parameters for the resulting end product, be it pulp or wood stock. It is therefore to be understood that, in operation of a refiner, a mixture of wood chips and water is delivered to the mass chamber and directed between at least two discs, engaged by the blades and dams thereof and, by friction, torn or ground into a pulp.
The manufacturing process employed for producing a refining disc is both show and complex. This is particularly so for an intermediate rotor disc, because it has two refining faces. One method of manufacturing a refining disc is to cast the disc (and blade pattern) as one solid piece. Typical materials for this method are carbon steel, iron or stainless steel. Regardless of material, the blades and dams must be machined or tooled in order to assure precise alignment, angular relationship and proper blade/dam configuration. A related method calls for the production of a frame consisting of two or more concentric rings interconnected by spaced, radially extending rods. The blades and dams are fixed, usually by welding, to the frame. Alternatively, a plurality of steel plates may first be fixed to the frame, and then the blade and dams may be fixed to the plates. Of course, for a double face disc the process is done twice. These methods have several disadvantages, many of which are explained in the commonly owned U.S. Pat. No. 3,614,826. Any foundry method of manufacture is a lengthy process. This particular foundry process is labor intensive, requiring skilled workmanship. Further, a refining disc made from cast iron or the like is very heavy. The sheer mass of the disc increases the refiner's operating costs and makes replacement difficult, causing long periods of down time. Specifically, the excessive weight of the disc wears on the motor utilized to rotate the disc. In addition, the steel casting process results in plates and blades that are porous and brittle.
Those skilled in the art will appreciate that the cutting action or refining efficiency of a refiner depends, in large part, on the number of blades; the more blades per given area, the stronger and more efficient the tearing action of the disc. Thus, another disadvantage with these prior art methods is that it is relatively impossible to cast a refiner disc with tall and/or thin blades spaced closely together or in complicated patterns. This results in poor stock distribution and processing. The foundry method also results in a loss of about 70% of the material used in the refining discs due to the wear on the blades.
Another method of manufacturing refiner discs was developed in response to these disadvantages. Specifically, refiner discs have been manufactured by welding stainless or carbon steel blades onto a base in the pattern or arrangement desired. To manufacture a two-sided rotor disc, blades are welded to both sides of the base. Specifically, the blades are individually welded along their bottom edges so as to be fixedly secured to the base. The completed disc, whether one-sided or two-sided, is secured within the refiner in the conventional manner.
The welding process is a lengthy process, requiring skilled workmanship. While not as heavy as cast iron refining discs, the discs manufactured according to the welding method are relatively heavy, which significantly contributes to the refiner's operating costs. The heat generated by the welding process softens the blades, resulting in a decrease in the useful life of the refining disc. Further, the blade-by-blade welding process is expensive in terms of materials, time and manpower.
Accordingly, there is a need for an improved rotor disc for a refiner that is less expensive to manufacture in terms of labor, time and materials costs; more efficient in terms of cutting strength, pulp stock distribution and fiber treatment, conserves energy and other refiner operating costs, and offers an increase in useful performance lifetime. Moreover, the preferred refiner disc would be relatively lightweight and easily installed.